FIG. 1 shows a typical optical lithographic fabrication system 100 for delineating features in a workpiece 120. Typically the workpiece comprises a semiconductor wafer (substrate), together with one or more layers of material(s) (not shown) located on a top major surface of the wafer. More specifically, monochromatic optical radiation of wavelength .lambda. emitted by an optical source 106, such as a mercury lamp, propagates through an aperture in an opaque screen 105, an optical collimating lens 104, a lithographic mask or reticle 103, and an optical focusing lens 102. The optical radiation emanating from the reticle 103 is focused by the lens 102 onto a photoresist layer 101 located on the top major surface of the workpiece 120. Thus, the pattern of the reticle 103--that is, its pattern of transparent and opaque portions--is focused on the photoresist layer 101. Depending upon whether this photoresist is positive or negative, when it is subjected to a development process, typically a wet developer, the material of the photoresist is removed or remains at and only at areas where the optical radiation was incident. Thus, the pattern of the mask is transferred to ("printed on") the photoresist layer 101. Subsequent etching processes, such as wet etching or dry plasma etching, remove selected portions of the workpiece 120. Portions of the workpiece 120 thus are removed from the top surface of the workpiece 120 at areas underlying those where the photoresist layer 101 was removed by the development process but not at areas underlying those regions where the photoresist remains. Alternatively, instead of etching the workpiece, impurity ions can be implanted into the workpiece 120 at areas underlying those where the photoresist layer was removed by the development process but not at areas underlying where the photoresist remains. Thus, the pattern of the mask 103--i.e., each feature of the mask--is transferred to the workpiece 120 as is desired, for example, in the art of semiconductor integrated circuit fabrication.
In fabricating such circuits, it is desirable, for example, to have as many transistors per wafer as possible. Hence, it is desirable to have as small a transistor or other feature size as possible, such as the feature size of a metallization stripe--i.e., its width W--or of an aperture in an insulating layer which is to be filled with metal, in order to form electrical connections, for example, between one level of metallization and other. Thus, for example, if it is desired to print the corresponding isolated feature having a width equal to W on the photoresist layer 101, a feature having a width equal to C must be located on the mask (reticle) 103. According to geometric optics, if this feature of width equal to C is a simple aperture in an opaque layer, then the ratio W/C=m, where m=L2/L1, and where m is known as the lateral magnification. When diffraction effects become important, however, the edges of the image become indistinct; hence the resolution of the features of the reticle 103, as focused on the photoresist layer and transferred to the workpiece, deteriorates.
In order to improve the resolution, in the prior art phase-shifting lithographic masks have been taught. A phase-shifting mask is used as the mask (or reticle) 103 in the system 100. Moreover, a phase-shifting mask comprises various regions that impart various phase shifts to the optical radiation that originates from the source 106 and propagates through the mask 103 in the system 103. More specifically, these phase-shifting masks have opaque and transparent regions. The transparent regions typically have at least two different thicknesses suitable for imparting at least two different phase shifts (each relative to the ambient atmosphere, typically air), typically equivalent to 0 and .lambda. radians, to the optical radiation (of wavelength .lambda.) propagating through the mask when it is being used as the reticle 103. The expression for the phase shift of a layer is given by (n-1)d, where n denotes the refractive index of the layer, d denotes the thickness of the layer, and the refractive index of the ambient is assumed to be equal to unity.
In order to facilitate repair of a phase-shifting mask structure, a variety of such mask structures has been proposed in prior art. A typical such phase-shifting mask structure 200 is depicted in FIG. 2. The structure 200 is an extension of the concepts taught in a paper by N. Hasegawa et al., entitled "Submicron Photolithography using Phase-Shifting Mask-Shifter Defect Repair Method," published in Fourth Hoya Photomask Symposium, Japan. This structure 200 includes a transparent quartz substrate 10, a pair of transparent silicon dioxide bottom and top phase-shifter (phase-shifting) layers 7 and 9, respectively, and a patterned opaque chromium layer 13. The patterning of the opaque layer 13 is in accordance with the desired optical image on the photoresist layer 101. A bottom etching end-point detection layer 6 and top etch-stopping layer 8, each typically made of tin oxide, respectively, separate the quartz substrate 10 from the bottom phase-shifter layer 7, and the bottom phase-shifter layer 7 from the top phase-shifter layer 9. The phase-shift introduced by the top phase-shifting layer 9 is equal to .pi. radian; the sum of the phase shifts introduced by the bottom phase-shifting layer 7 and the bottom end-point detection layer 8 is also equal to .pi. radian.
The structure 200 contains protruding and indentation defect regions 21 and 22, respectively. The purpose of the bottom end-point detection layer 6 is to enable repair of the mask structure 200--insofar as the indentation defect region 22 is concerned--by a process of etching (ion milling) the defect region (and perhaps its neighborhood) down to the bottom end-point detection layer 6 without (undesirably) penetrating farther downward, as explained more fully below. The purpose of the top etch-stopping layer 8 is to enable patterning, as by anisotropic dry etching, of the top phase-shifter layer 9 without the etching undesirably penetrating down into the bottom phase-shifting layer 7. In addition, the top etch-stopping layer 8 can serve the function of end-point detection during repair of the mask structure 200--insofar as the protruding defect region 21 is concerned--by a process of etching (ion milling) without penetrating farther downward than the bottom surface of the defect region 21 itself.
In order to repair the mask structure 200--i.e., to remove unwanted phase-shifting effects caused by the defect regions 21 and 22--first the protruding defect region 21 can be removed, as by focused ion beam etching (ion milling) that scans the top surface of the top etch-stopping layer 8. The milling is terminated as soon as secondary ions or secondary electrons being emitted and detected during the ion milling begin to shift from those known to be emitted by the material of the defect region 21 to those known to be emitted by the material of the top etch-stopping layer 8. Then the indentation defect region 22 is removed by anisotropically (vertically) etching it, together with a neighborhood of it, through both the phase-shifting layers 9 and 7 as well as the top etch-stopping layer 8, down to the top surface of the bottom end-point detection layer 6 but not at all penetrating it. In this way, the resulting phase shift associated with the resulting hole penetrating through the bottom and top phase shifter layers 7 and 9 plus the top etch-stopping layer 8 is equal to .pi. radian+.pi. radian=2.pi. radian-- i.e., equivalent to a zero phase shift, as is desired.
The foregoing technique suffers from one or more of the following shortcomings. First, the optical absorptions by the top etch-stopping layer 8 and of the bottom end-point detection layer 6 can be undesirably high, especially if the wavelength .lambda. is in the deep ultraviolet (typically the 248 nm optical wavelength emitted by an excimer laser source), and hence the overall optical transmission of the structure 200 can become undesirably low. Second, the refractive index discontinuities at various interfaces between the different materials of the various layers in the structure 200 can give rise to undesirable high-amplitude optical reflections, which again can result in undesirably low overall optical transmission. Third., the complexity of fabrication, owing to the etch-stopping layer 8 and the bottom end-point detection layer 6, can undesirably add to production costs, and also can lower production yields because of unwanted pinholes that can exist in these layers. Therefore, it would be desirable to have a phase-shifting mask structure for use in the system 100 that mitigates the shortcoming of this prior art.